getdtablesize.c

/* getdtablesize() function for platforms that don't have it. Copyright (C) 2008-2010 Free Software Foundation, Inc. Written by Bruno Haible <bruno@clisp.org>, 2008. This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */#include <config.h>/* Specification. */#include <unistd.h>#if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__#include <stdio.h>/* Cache for the previous getdtablesize () result. */staticint dtablesize;
int
getdtablesize (void)
{
if (dtablesize == 0)
{
/* We are looking for the number N such that the valid file descriptors are 0..N-1. It can be obtained through a loop as follows: { int fd; for (fd = 3; fd < 65536; fd++) if (dup2 (0, fd) == -1) break; return fd; } On Windows XP, the result is 2048. The drawback of this loop is that it allocates memory for a libc internal array that is never freed. The number N can also be obtained as the upper bound for _getmaxstdio (). _getmaxstdio () returns the maximum number of open FILE objects. The sanity check in _setmaxstdio reveals the maximum number of file descriptors. This too allocates memory, but it is freed when we call _setmaxstdio with the original value. */int orig_max_stdio = _getmaxstdio ();
unsignedint bound;
for (bound = 0x10000; _setmaxstdio (bound) < 0; bound = bound / 2)
;
_setmaxstdio (orig_max_stdio);
dtablesize = bound;
}
return dtablesize;
}
#endif